This invention relates to a scroll type fluid displacement apparatus, and more particularly, to an axial thrust load supporting mechanism for an orbiting scroll of a scroll type fluid displacement apparatus.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 issued to Creux discloses such apparatus which includes two scrolls each having a circular end plate and a spiroidal or spiral involute element. The scrolls are maintained angularly and radially offset so that both spiral elements interfit to form a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion. Thus, scroll type fluid displacement apparatus may be used to compress, expand or pump fluids.
Generally, in conventional scroll type fluid displacement apparatus, one scroll is fixed to a housing and the other scroll is an orbiting scroll. The orbiting scroll normally is eccentrically supported on a drive or crank pin of a drive shaft, which causes orbital motion of the orbiting scroll upon rotation of the drive shaft. Also, a conventional scroll type fluid displacement apparatus includes a rotation preventing mechanism which prevents rotation of the orbiting scroll to thereby maintain the fixed and orbiting scrolls in a predetermined angular relationship during operation of the apparatus.
Because the orbiting scroll in conventional scroll type fluid displacement apparatus normally is supported on the crank pin in a cantilever manner, an axial slant of this orbiting scroll naturally occurs. Axial slant also occurs because the movement of the orbiting scroll is not rotary motion around the center of the orbiting scroll, but is orbiting motion caused by the eccentric movement of the crank pin as this crank pin is driven by the rotation of the drive shaft. Several problems result from the occurrence of this axial slant including improper sealing of line contacts, vibration of the apparatus during operation and noise caused by physical striking of the spiral elements. One simple and direct solution to these problems is the use of a thrust bearing device for carrying the axial loads. Thus, a conventional scroll type flid displacement apparatus usually is provided with a thrust bearing device.
One recent attempt to improve the rotation preventing and thrust bearing devices in scroll type fluid displacement apparatus is described in U.S. Pat. No. 4,160,629 (Hidden et al.) and U.S. Pat. No. 4,259,043 (Hidden et al.) In the apparatus of these patents, the rotation preventing and thrust bearing functions are integral with one another. A rotation preventing/thrust bearing mechanism according to these patents includes one set of identations formed on the outer end surface of the circular end plate of the orbiting scroll and a second set of indentations formed on the end surface of a fixed plate attached to the housing of the apparatus. A plurality of balls or spheres are placed between the indentations formed on these surfaces to carry the axial load and prevent rotation of the orbiting scroll.
In the above described rotation preventing/thrust bearing device, the maximum orbital radius of this rotation preventing/thrust bearing device is defined by factors such as diameter of the balls, diameter of the indentations formed on each surface and the displacement of the balls in the indentations, whereas the orbital radius of the orbiting scroll is defined by the number of turns of the spiral element. Nevertheless, the orbital radius of both the rotation preventing/thrust bearing device and the orbiting scroll should be the same to effectively perform the function of rotation prevention. However, because of dimensional errors caused by the manufacture of parts and the assembly of the apparatus, the orbital radius of the rotation preventing/thrust bearing device must be made larger than the orbital radius of the orbiting scroll to maintain the seal of the fluid pockets. Unfortunately, this variance in the orbital radius due to manufacturing tolerances results in the disadvantage of too much play in the movement of the balls within the device.
Furthermore, the moment of rotation (.rho.) of the orbiting scroll, which is in the same direction as the rotating drive shaft, is defined by the following formula: ##EQU1## wherein Fg is the resultant force of the gas pressure acting on the spiral element and r.sub.o is the distance between the center of the fixed scroll and the center of the orbiting scroll (hereinafter called the "crank radius"). Though this moment of rotation acts on the rotation preventing/thrust bearing device, occasionally the direction of the moment is offset from the rotating direction of the drive shaft due to changes in the gas pressure force. When this occurs in the above described rotation preventing/thrust bearing device, because of the play of the balls in this device, the direction of force acting on the balls changes which causes vibration.
Referring to FIGS. 1 and 2, the above phenomenon will be described. FIG. 1 is a diagrammatic sectional view illustrating the relationship of conventional fixed and orbiting scroll. FIG. 2 is a pressure distribution graph illustrating the pressure in sealed off pockets taken along line A--A in FIG. 1. In conventional scrolls, the number of turns of the spiral elements in both scrolls is the same, i.e., the fixed and orbiting scrolls are formed as mirror images. As a result, as shown in FIG. 2, the gas pressure distribution is symmetrical about the midpoint of crank radius r.sub.o. Thus, the resultant force of gas pressure Fg acts on the midpoint of crank radius r.sub.o perpendicular to the direction of crank radius r.sub.o. The force Fd, which is the same vector force as the resultant force Fg but in the opposite direction, acts on the center of the orbiting scroll to balance the resultant force Fg. Therefore, forces Fg and Fd create the moment of rotation or a rotating force defined by Fg.times.r.sub.o /2. This rotating force is dependent on changes in gas pressure so that, in the rotation preventing/thrust bearing device described above, play of the balls occurs within the rotation preventing/thrust bearing device resulting in undesirable vibration and noise.